1. Technical Field
The present disclosure relates to a toner, and an image forming method and a process cartridge using the toner.
2. Description of Related Art
In the field of electrophotography, recently, toner is required to be fixable at much lower temperatures for the objective of saving energy as well as meeting demands for improving printing speed and image quality.
Generally, as the printing speed of an electrophotographic image forming apparatus increases, the resulting image quality decreases mainly because a defective fixation of toner occurs.
In the process of fixing toner (hereinafter the “fixing process”), a toner image is fixed on a recording medium, such as paper, by application of heat and pressure. When the printing speed gets higher, the toner image is supplied with less heat energy and is defectively fixed on the recording medium. The defectively-fixed toner image may have a rough surface or may generate a residual image (this phenomenon is hereinafter called as “cold offset”). Such deterioration of the toner image caused by a high printing speed may be prevented by increasing the fixing temperature. However, increasing the fixing temperature is not the best solution because the high fixing temperature adversely affects the other image forming processes, accelerates deterioration of the fixing members, and increases consumption energy.
In view of this situation, toner itself is required to improve the fixing performance, i.e., to be fixable at much lower temperatures, especially in high-speed image forming apparatuses.
One attempt to improve the fixing performance of toner involves controlling thermal properties, such as the glass transition temperature (Tg) and the softening temperature (T1/2), of its binder resins. However, lowering Tg may cause deterioration of heat-resistant storage stability and lowering T1/2 (e.g., lowering the molecular weight of the binder resins) may cause the hot offset problem. Merely controlling thermal properties of the binder resins does not provide a toner having a good combination of low-temperature fixability, heat-resistant storage stability, and hot offset resistance.
JP-S60-90344-A, JP-S64-15755-A, JP-H02-82267-A, JP-H03-229264-A, JP-H03-41470-A, and JP-H11-305486-A each propose polyester binder resins, having low-temperature fixability and heat-resistant storage stability, in place of styrene-acrylic binder resins having been widely used so far.
JP-S62-63940-A proposes a non-olefin-based crystalline polymer binder which sharply melts at the glass transition temperature, for improving low-temperature fixability.
JP-2931899-B2 (corresponding to JP-H11-249339-A) and JP-2001-222138-A each propose crystalline polyester binders which sharply melt, for improving low-temperature fixability.
The crystalline polyester described in JP-2931899-B2 has a low acid value of 5 or less and a low hydroxyl value of 20 or less.
JP-2004-46095-A describes a toner having a sea-island phase separation structure formed of a crystalline polyester resin and an amorphous polyester resin which are incompatible with each other.
JP-2007-33773-A describes a toner within which a crystalline polyester resin is properly dispersed and having a specific endothermic profile determined by differential scanning calorimetry, for giving low-temperature fixability and heat-resistant storage stability to toner.
JP-2005-338814-A describes a toner including a relatively large amount of a crystalline polyester resin.
JP-4118498-B2 (corresponding to JP-2002-082484-A) describes a toner having a specific molecular weight distribution, including a certain amount of chloroform-insoluble contents, and including two or more kinds of binder resins each having different softening temperatures.
JP-2007-206097-A describes a toner including a crystalline polyester resin and an amorphous resin in which a ratio of the heights of peaks specific to the crystalline polyester resin and the amorphous resin determined by a Fourier transform infrared spectroscopy total reflection method after the toner is stored in a thermostatic chamber at 45° C. for 12 hours.
In a process called developing process, toner particles having been charged in a developing unit are transferred onto a latent image formed on an image bearing member so that the latent image is developed into a toner image. Depending on the moving speed of the image bearing member, for example, when the moving speed of the latent image bearing member is relatively high, the developing unit may employ multiple magnetic developing rollers so as to extend the developing area as well as the developing time period.
The developing unit employing multiple magnetic developing rollers (hereinafter “multistage developing unit”) has a higher developing ability than that employing only one developing roller, and can be applicable to large-area-image printing while improving image quality. Additionally, in such a multistage developing unit, the toner content in a two-component developer can be reduced and the rotational speed of the developing rollers can be reduced. As a result, the occurrence of toner scattering and carrier deterioration is prevented and the lifespan of the two-component developer is extended.
JP-2011-100106-A describes a toner including a crystalline polyester.